1. Field of the Invention
This invention relates to process control systems used to maintain a process parameter at a predetermined value or level, herein called the set point value. A measurement signal indicative of the present value of the parameter to be controlled is compared to a set point value signal. An error signal, related to the difference therebetween, is generated and used to adjust a final operator such as a valve, cooperating with the process to vary the parameter.
The set point value source is typically a manually adjustable potentiometer connected to a D.C. voltage source. Even in installations in which the set point value source is provided by other means, such as a secondary controller in cascade configuration, or a digital computer in a Supervisory Control configuration, there is almost always provided a manual adjust mechanism to allow the human process operator to manually change the set point value. In the majority of such installations a set point value indicator is coupled to the set point value source to indicate the present value of the set point as an aid to the operator. In many cases the measurement signal is also displayed on the same scale for the operator's convenience.
2. Description of the Prior Art
In process control installations in which the set point value is indicated, it is extremely important that the indicated value be accurate. The difference between the indicated set point value and the indicated measurement value dictates the proper action to be taken by the process operator during initial start-up, modification of the process, or while under emergency conditions. It is therefore necessary to the safe operation of process installations to assure that the set point value is indicated accurately.
A conventional approach used to assure the accuracy of the set point value indication at a reasonable cost is to mechanically couple the set point value source to a drive wheel of the indicator assembly and couple the pointer to the drive wheel by a toothed belt. A thumbwheel is mounted on a slider, usually with an idler wheel, and the slider is biased out of contact with the drive wheel. Manual actuation of the thumbwheel moves the slider to allow the idler wheel to contact the drive wheel and transfer motion of the thumbwheel thereto. In this manner the set point value source and the set point value indicator are simultaneously adjusted. Resistance to motion exhibited by the pointer is allowed to cause slippage between the idler wheel and the indicator drive wheel, to prevent damage to the toothed belt and/or dislocation between the actual and indicated set point values.
In such conventional designs, however, increased thumbwheel pressure can overcome this frictional slippage and cause dislocation by continuing to drive the set point value source without causing a corresponding change in the set point value indicator.